196 BULLETIN OF THE 



broken by an eccentric protuberance on one side. Both mouth and anus open 

 through the lower hemisphere ; the former on one side just below the upper of 

 the equatorial ciliated bands, and the latter on the eccentric prominence already 

 mentioned. The whole exterior surface of the body is ciliated, and the equa- 

 torial band consists of two parallel rings, which extend side by side for most of 

 their course, and separate from each other in the region of the mouth, the 

 larger passing above and the smaller below this orifice. This position of the 

 two ciliated rings in an older larva can be seen in Loven's original figures of 

 the European larva. The upper hemisphere has its walls unbroken by any ori- 

 fice, and is of regular dome-like shape. At its pole arises a short flagellum' (/), 

 which waves back and forth as the larva moves thi'ough the water. The flagel- 

 lum found at its apex is an embryonic structure, and appears to be lost in 

 more advanced larvae. On the body walls just below the pole, in a granulai 

 zone surrounding the point from which the flagellum arises, there is a collection 

 of cells more or less scattered, the thickness of which decreases more and more 

 near the ciliated equatorial band. This granular mass is the first appearance of 

 that nervous centre which later aggregates about this pole of the larva, and 

 bears the ocelli. Slightly removed from the base of the flagellum, in the walls 

 of the larva, are two pigment spots, one on each side, with well-marked lens, 

 which as the larva grows older and older approximate closer and closer, until 

 they are brought into the immediate vicinity of the pole, in connection with 

 the consolidated collection of cells already mentioned. In the youngest larva, 

 however, they are widely separated ; later, they grow nearer each other. Two 

 prominent clusters of cells (c d) are found in the lower hemisphere in the walls 

 diametrically opposite the cluster on the upper hemisphere. These also later 

 form a nervous centre at this extremity of the worm. Around the rim of the 

 larva, in the slight ring-shaped protuberance from which the cilia arise, many 

 brown and yellow colored cellular bodies (Fig. 10", p) of a problematical char- 

 acter can be found. These bodies vary somewhat in color and size, being gen- 

 erally brownish, yellowish, or greenish, and have a regular spherical shape. 

 They appear to be suspended in membranous dilatations of the disk rim, and 

 to hang from it by a slight attachment. Their disposition on the bell margin 

 is iiTegular, and not constant. Can this be an instance of the symbiosis of an 

 algous growth on a worm larva ? 



The interior of the larva is occupied for the most part by three cavities, each 

 with its own walls separate from those of the body cavity. Two of these have 

 external openings into the medium in which the larva lives. The greater part 

 of the interior of the larva is occupied by the largest of these three organs, 

 which is called the stomach. This structure, which is almost globular in 

 shape, has two openings, one into each of the two smaller ca , ities. It does not 

 communicate with the surrounding water, and its whole inner surface is cili- 

 ated. Of the remaining two mternal sacs, one is an oesophagus, and the other 

 an intestine. The oesophagus opens externally through the mouth, and inter- 

 nally into the stomach. Its walls are muscular, contractile, and the inner 

 surface is ciliated. Granules of food in masses were observed in its cavitJ^ 



